Relief image printing plates are used in both flexographic and letterpress processes for printing on a variety of substrates, including paper, corrugated stock, film, foil, and laminates. The photocurable elements that are used to make relief plates typically include a support layer and one or more layers of photocurable polymer in the form of solid sheets. The printer typically peels a cover sheet from the element to expose the photocurable polymer and places a silver halide photographic negative or some other masking device upon the photopolymer. The negative-bearing photocurable element then is exposed to ultraviolet (UV) light through the negative, thereby causing exposed areas of the element to harden, or cure. After the uncured areas of the element are removed, cured polymer remains as the relief printing surface.
Corrugated boxes and other, relatively large objects that are printed using relief image printing plates often bear actual printing on only a small portion of their total surface area. One way to print such an object is to prepare a single relief image plate having a surface area corresponding to the total surface area of the object. Since only a portion of the object's surface needs to be printed, however, only a portion of the relief image plate will actually used for ink transfer. The remainder of the plate will be unused and, essentially, wasted.
To minimize such waste, those skilled in the art often print relatively large objects with composite printing plates that are prepared by mounting a plurality of relief image printing plates on a common carrier sheet. The individual plates, however, are mounted only on those portions of the carrier that correspond to the portions of the object that actually need to be printed. Although such composite plates do minimize waste, the current system for mounting their constituent relief image plates is laborious and requires careful adhesion of the plates to the carrier while assuring registration to within 0.005 inches on-press for high quality printing and multi-color reproduction. For multi-color reproduction, wherein a single plate is used for printing each of the individual colors, accurate registration of the plates with respect to one another is crucial.
Consequently, there remains a need in the art for alternative processes for preparing composite printing plates. In particular, there remains a need for alternative processes for accurate registration of the constituent relief image printing plates or for processes wherein accurate registration is not necessary.